1. Field of the Invention
The invention relates to a recursive noise reducer which is suitable when applying to an image signal in which, for example, a coding using a DCT is decoded.
2. Description of the Related Art
A recursive noise reducer using a frame memory is known in the field of digital image signal processes. The noise reducer is constructed in a manner such that a portion of small level in a difference between an input image signal and a video signal one frame before from the frame memory is extracted as a noise and the extracted noise component is subtracted from the input image signal, so that the noise is reduced and the signal in which noises are reduced is written into the frame memory. If a field memory is used in place of the frame memory, a capacity of the memory can be decreased.
The DCT (Discrete Cosine Transform) is used in an image compression coding of an MPEG (Moving Pictures Expert Group) standard or the like. The DCT divides an image into blocks and performs a cosine transformation coding every block. In case of the DCT, a block distortion in which a boundary of the block is seen in the decoded image occurs due to an quantization error of coefficients of an AC component. Since the block distortion differs between the fields, low band noises between fields in which a time dependent transition of the block distortion is seen as a flicker occur.
In the conventional recursive noise reducer, all of the image data of one field or one frame is written in a memory in a feedback loop. In other words, since a noise reducing process is performed to all of the bands in which an image signal exists, a memory capacity cannot be reduced to a value smaller than one field or one frame, so that there is a problem of high costs as a home-use apparatus. A system in which in the case where an image signal obtained by decoding an image signal which was DCT coded is inputted, a block distortion is predicted and the block distortion is corrected upon decoding has also been proposed. However, the block distortion cannot correctly predicted depending on conditions of an image and there is a possibility of an erroneous operation.